> Feels like a low sample size

Its not a sample, it is the whole universe of analysis. (If you treat it as a sample of, say, US drivers killed in accidents in the same period, then errors due to sample size are probably the least of its problems.)

We don't know that. We don't even know if there's selection bias.

The article says the research was "focusing on 246 deceased drivers who were tested for THC", and that the test usually happens when autopsies are performed. It doesn't say if autopsies are performed for all driver deaths, and it also doesn't say what exactly is "usually".

If (for example) autopsy only happens when the driver is suspected of drug use, then there's a clear selection bias.

Note that this doesn't mean the study is useless: they were able to see that legalization of cannabis didn't have impact on recreational use.